This invention relates to a method of making a catalyst in which small catalyst particles are dispersed on the surface of larger catalyst carrier particles. More specifically, it relates to using a dry-coating process to coat nanometer-sized catalyst particles on the surface of larger catalyst carrier particles. The dry-coated catalyst particle/carrier particle composite mixture is then adapted for a catalyst application, such as automotive exhaust gas treatment or for a fuel cell reformer.
Catalysts are used in many different applications. Their compositions and structures vary, as do the processes by which they are prepared. In applications where the catalyst used is present as a separate phase from the reacting chemical species that it contacts, i.e., heterogeneous catalysis, a method must be employed to support the catalytic entity in the presence of the reacting phase. Automotive exhaust treatment catalysts are an example of heterogeneous catalysts, and the background for this invention will be illustrated in that context.
Automotive vehicles have used catalytic converters to treat unburned hydrocarbons, carbon monoxide and various nitrogen oxides produced from the combustion of hydrocarbon fuels in the engine. The engine exhaust gases flow through a catalytic converter that contains a very small amount, e.g., an ounce, of noble metals, such as palladium, platinum and rhodium. The catalytic converter comprises a stainless steel can that houses an extruded ceramic body, such as cordierite composition, in the shape of an oval honeycomb, generally referred to as a monolith. The extruded body contains several hundred small longitudinal passages per square inch of its cross-section. The engine exhaust passes through these channels, contacts the catalyst coated thereon, and the hazardous constituents are oxidized and/or reduced.
The catalyst combination coated on the walls of the monolith passages, or channels, comprises particles of activated alumina, or the like, which carry much smaller and dispersed particles of the noble metals. A challenge in preparing such exhaust treatment catalysts lies in making maximum use of the relatively expensive noble metal. The noble metal must be distributed so that all of it, or nearly all of it, is exposed to contact the exhaust gas. Thus, catalyst particles are distributed on carrier particles and this combination supported on the walls of the monolith for contacting the exhaust gas.
In accordance with present practice, an aqueous slurry of activated alumina is first applied as a thin film on the walls of the monolith passages. Activated alumina is a material that is processed to have a very large surface area per unit mass/volume. To enhance its catalyst carrier properties, the alumina may contain small amounts of other metal oxides, such as cerium oxide and lanthanum oxide. The aqueous slurry of finely divided carrier particles is drawn through the channels of the monolith and the excess drained off. The coated monolith is dried, and the coating calcined. A thin layer of alumina catalyst carrier particles is thus fixed to the walls of the channel.
The noble metal(s) to be coated on the alumina is prepared as a water-based solution of suitable salts. This solution is used to soak and impregnate the alumina coating, thus permitting the noble metal compounds to infiltrate the irregular surface of the alumina particles that provides its remarkably large area. Residual solution, containing the noble metal, if any is removed. This impregnation of the alumina with a noble metal solution is referred to as a wet process. The noble metal/alumina coating on the walls of the monolith passages is known as a wash coat of the catalyst.
Exhaust catalysts prepared in this way have worked well for many years. However, losses and inefficiencies remain from using this wet processing. Moreover, if possible, there is a great need to disperse the expensive metal even further so that smaller quantities can be used and/or even more complete elimination of undesirable exhaust products can be obtained.
This invention uses nanometer-sized particles of catalytic noble metals, such as platinum, and certain metal oxides, such as copper oxide and zinc oxide. These metals and metal oxides are commercially available in batch quantities in size ranges of, for example, 5-50 nanometers. Since such particles are usually somewhat irregular in shape, xe2x80x9csizexe2x80x9d means the xe2x80x9cdiameterxe2x80x9d of a particle or like or equivalent characteristic linear dimension. In general, catalyst particle lots where the particle size distribution is within the size range of about 1 to 500 nanometers are suitable for use in the practice of this invention. A special feature of this invention is a process of dispersing, or coating, such small catalytic particles on the surfaces of larger catalyst carrier particles. The preferred carrier particles include, but are not limited to; high surface area alumina particles and alumina that incorporates metal oxides, such as cerium and lanthanum.
The coating process of this invention yields high effective surface area of catalyst particles on the catalyst carrier particles. In the process, the catalyst particles and catalyst carrier particles are mechanically mixed under conditions under which they impact each other and most of the catalyst particles surprisingly end up adhering to the surface of the larger carrier particles. The process works most favorably when the carrier particles are substantially larger than the catalyst particles. Preferably, the median diameter, or other characteristic dimension, of the carrier particles is at least ten times the median diameter of the catalyst particles to obtain the catalyst particle-on-carrier particle composite material. The median particle diameter is suitably determined by the dynamic light scattering particle size measurement method. While given batches of both catalyst and carrier particles will have ranges of dimensions, it is preferred that there be suitably large carrier particles in the mix for each catalyst particle. However, as illustrated in Example 2 below, smaller (less than 10xc3x97 catalyst particle size) carrier particles can be used to make the composite material when they are first coated on a larger support material such as micrometer sized ceramic fibers.
The coating process is a dry process. It does not require the use of water or any other constituent to accomplish the coating of the catalyst particles on their carrier particles. Often the nanometer sized catalyst particles are initially present in clusters or agglomerates. The suitable dry mixing processes break up these agglomerates and disperse the catalyst particles on the carrier particles. The resulting catalyst particle/catalyst carrier combination evidences uniform and well-dispersed catalyst particles on the carrier particles.
The catalyst particle/carrier particle composite, thus produced, is in the form of a powder. In many applications, application of this powder to a suitable support structure will be necessary. For example, the composite powder could be slip coated on the walls of the channels of a corderite monolith for the treatment of automotive exhaust gas. In other applications, the composite powder could be coated on ceramic fibers, carbon fibers, or other kinds of catalyst support structures. For these applications, the carrier particles could be coated on the support bodies and the catalyst particles later coated on the carrier particles.
Regardless of the support means for the composite catalyst/carrier particles, this invention provides a simple and dry coating process by which nanometer-sized catalyst particles are dispersed on the surfaces of larger catalyst carrier particles in a form in which the catalyst surfaces can be effectively utilized. In essence, suitable quantities of catalyst particles and carrier particles are incorporated in the mixer and the mixed product is useful as is without recycling or purification.
Other objects and advantages of this invention will become apparent from a detailed description of specific embodiments that follow.